Multicopter measurements of volcanic gas emissions at Masaya (Nicaragua), Turrialba (Costa Rica) and Stromboli (Italy) volcanoes: Applications for volcano monitoring and insights into halogen speciation

Volcanoes are a natural source of several reactive gases (e.g. sulfur and halogen containing species), as well as nonreactive gases (e.g. carbon dioxide). Besides that, halogen chemistry in volcanic plumes might have important impacts on atmospheric chemistry, carbon to sulfur ratios and sulfur dioxide fluxes are important established parameters to gain information on subsurface processes. In this study we demonstrate the successful deployment of a multirotor UAV 20 (quadcopter) system with custom-made lightweight payloads on board for the compositional analysis and gas flux estimation of volcanic plumes. The various applications and their potential with such new measurement strategy are presented and discussed on example studies at three volcanoes encompassing flight heights of 450 m to 3300 m and various states of volcanic activity. Field applications were performed at Stromboli Volcano (Italy), Turrialba Volcano (Costa Rica) and Masaya Volcano (Nicaragua). Two in-situ gas-measuring systems adapted for autonomous airborne measurements, based on electrochemical 25 and optical detection principles, as well as an airborne sampling unit, are introduced. We show volcanic gas composition results including, abundances of CO2, SO2 and halogen species. The new instrumental set-ups were compared with established instruments during ground-based measurements. For total SO2 flux estimations a small differential optical absorption spectroscopy (DOAS) system measured SO2 column amounts on transversal flights below the plume, showing the potential to replace ground-based manned operations. 30 At Stromboli volcano, short-term fluctuation of the CO2/SO2 ratios could be determined and confirm an increased CO2/SO2 ratio in spatial and temporal proximity to explosions by airborne in-situ measurements. Reactive bromine to sulfur ratios of 0.19 x 10 to 9.8 x 10 were measured in-situ in the plume of Stromboli volcano downwind of the vent. Atmos. Meas. Tech. Discuss., https://doi.org/10.5194/amt-2017-335 Manuscript under review for journal Atmos. Meas. Tech. Discussion started: 28 November 2017 c © Author(s) 2017. CC BY 4.0 License.


Introduction
Gaseous volcanic emissions consist of a variety of different compounds and are dominated by water vapor (H2O), carbon dioxide (CO2), sulfur dioxide (SO2), and hydrogen sulfide (H2S) (Symonds et al., 1994).Minor abundant, but nonetheless important gas species are halogen-bearing compounds which are emitted as hydrogen halides (HF, HCl, HBr and HI) and later partly transformed by heterogeneous reactions into other halogen species, such as bromine monoxide (BrO) or chlorine dioxide (OClO) (Bobrowski et al., 2007).The relative gas composition varies with the types of volcanoes and magmas as well as with transport and degassing mechanisms.Changes in the magma degassing behavior and/or the hydrothermal systems beneath volcanoes generally influence the gas composition and gas fluxes.Measuring the emitted gas composition can provide crucial information on understanding subsurface processes related to activity changes (e.g.Allard et al., 1991;Aiuppa et al., 2007;Bobrowski and Giuffrida, 2012;de Moor et al., 2016a;Liotta et al., 2017) and help to estimate fluxes of the geological carbon cycle (e.g.Burton et al., 2013) and tectonic processes controlling volcanic degassing (e.g.Aiuppa et al., 2017).In the field of volcanic monitoring, the observation of gas composition changes became an important tool for detecting precursory processes for volcanic eruptions.It has been shown that enhanced CO2/SO2 emission ratios appear prior to eruptive volcanic activity within the timescale of hours to weeks (e.g.Giggenbach, 1975;Aiuppa et al., 2007;de Moor et al., 2016b).In-situ measurements of this gas ratio has become a well-established method using electrochemical (SO2) and infrared (CO2) sensors, implemented in so-called Multi-GAS (MG) instruments, which may also contain other sensors and are field-deployable to work autonomously close to volcanic emission sources (Shinohara, 2005;Aiuppa et al., 2006).
Another important parameter for the characterization of volcanic activities are emission rates (fluxes).Particularly, the determination of SO2 fluxes has become a standard procedure by traversing the plume and multiplying the integrated SO2 cross-section with the estimated plume transport speed (e.g.McGonigle et al., 2002;Galle et al., 2003;López et al., 2013).
With the development of small DOAS instruments, traversing the plume is not only feasible by cars, boats, or manned aircrafts, but also by walking in case of poorly accessible terrains.
Furthermore, knowledge about in-plume chemical reactions can be drawn from compositional assessment of the gases, which also helps understanding their impact on atmospheric chemistry (e.g. Lee et al., 2005;von Glasow, 2010;Gliß et al., 2015).
Especially the halogen chemistry is of great interest as BrO/SO2 ratios in volcanic plumes are readily measurable by remote sensing UV spectrometry and have been discussed in recent years as another potential precursory observable for volcanic activity changes.It was observed that the BrO/SO2 ratio decreases in advance to eruptive phases (Lübcke et al., 2014) and is lower during periods of continuous activity (Bobrowski and Giuffrida, 2012).However, BrO is not a directly emitted species rather than the product of complex heterogeneous chemistry in the volcanic plume involving reactions with magmatic gases with entrained air (e.g.Gerlach, 2004, Bobrowski et al., 2007).The variation of BrO by plume age and a transversal distribution in the plume for this species was observed by differential optical absorption spectroscopy (DOAS) measurements (Bobrowski and Platt, 2007).Furthermore, other reactive halogen species with oxidation states ≠ -1 (e.g.Br2, Cl2, BrCl and others) have been measured in-situ in the plume of Mt.Etna, Italy (Rüdiger et al., 2017) and Mt Nyamuragira (Bobrowski et al. 2017, Atmos.Meas.Tech.Discuss., https://doi.org/10.5194/amt-2017-335Manuscript under review for journal Atmos.Meas.Tech.Discussion started: 28 November 2017 c Author(s) 2017.CC BY 4.0 License.accepted).In the last decade, several model studies (e.g.Bobrowski et al., 2007, Roberts et al., 2009;von Glasow, 2010;Roberts et al., 2014;Jourdain et al., 2016) have engaged on the variation of halogen variability in volcanic plumes with respect to various atmospheric and magmatic parameters.In the case of bromine, it was modelled that the initial emitted hydrogen bromide is depleted shortly after emission under consumption of tropospheric ozone and is transformed to reactive species such as BrO, HOBr, Br2, BrCl and BrONO2.Due to the challenging task of accessing volcanic plumes on a timescale of minutes after emission, and the lack of spectroscopic methods for most of these reactive species, uncertainties about their relative abundances still exist.One approach towards the in-situ observation of reactive halogen species is the application of gas diffusion denuder sampling using a selectively reactive organic coating (1,3,5-trimethoxybenzen, TMB) to trap and enrich gaseous species containing a halogen atom with the oxidations state +1 or 0 (e.g.Br2 or BrCl), while being insensitive to the particle phase (Rüdiger et al., 2017).
In the case of most volcanoes sampling on the crater rim presents great logistical challenges and hazards for people and instruments.During phases of high activity crater rims are usually not accessible at all and even during quiescent degassing work at the crater rim represents a considerable risk.However, the knowledge of plume gas composition is an important component for activity assessments of volcanoes (e.g.Carroll and Holloway, 1994;Aiuppa et al., 2006) and therefore gas monitoring stations are deployed and maintained at the crater rim by researchers, putting themselves at risks.Advancements in the application of remote sensing techniques have helped to minimize personnel exposition to the volcanic danger zone (e.g.Galle et al., 2003;Tamburello et al., 2011).However, still today the detection of certain gas species and/or total amounts of all species of an element is not possible remotely (neither ground based nor with satellites) and therefore in-situ measurements are an important tool, especially for resolving chemical reactions and speciation changes in aging volcanic plumes.With an in-situ sampling strategy, obtaining samples from the freshly emitted plume is feasible, while ground-based in-situ sampling of the aged plume further downwind is rarely possible and dependent on specific wind and geographical conditions.In the last decade with the development of compact and cost effective unmanned aerial vehicles (UAV) several deployments of gas sensors and other in-situ methods (e.g.particle detection (Altstädter et al., 2015)) as well as applications of spectrometers were realized (e.g.McGonigle et al., 2008, Diaz et al., 2015, Mori et al., 2016, Villa et al., 2016 and references therein).While these applications focused mostly on the use of sensors and spectroscopy methods, sampling (e.g.canister sampling (Chang et al., 2016)) of the plume has not been reported for volcanoes.Here we present a low-cost UAV-deployable sampling (gas diffusion denuder) and sensing (electrochemical/optical sensors) systems for the determination of CO2, SO2 and halogen species.Our system enabled us to access the plume close to an active vent as well as the aged plume several km downwind of the source and elevated from ground, without exposing operators to the risks in proximity to active vents or employing manned aircrafts to potential engine-damaging ash and gas plumes.In addition to that, the UAV-deployment of a lightweight DOAS instrument for SO2 flux estimations is presented herein, which enables fast plume traversing in terrains that are usually not accessible by cars or even by foot.

Stromboli
Stromboli volcano, the northernmost island of the Aeolian volcanic arc (Italy), rises 924 m above sea level (a.s.l.).The island represents the top part of a large 2500-m-high stratovolcano emerging from the Tyrrhenian Sea floor.Stromboli is well known for its regular (~ every 10-20 min) explosive activity (Strombolian activity).Intermittently, continuous passive degassing occurs from the active vents, which are located in the so-called crater terrace at about 750 m a.s.l.. Ejected lava material is dominantly deposited in a northwestern direction, forming a hardly safe to non-accessible horseshoe-shaped area (Sciara del Fuoco).The summit above the craters is well accessible and characterized by a numerous amount of monitoring stations continuously observing the activity.Thus, Stromboli has been a laboratory volcano for studying magma degassing processes (Allard et al. 2008) and field-testing new instrumentations for many years.
While most gas monitoring station positions benefit from a north westerly wind direction, a south easterly wind only allows gas plume detection by spectroscopic methods.In this study, due to the dominance of southeast winds in early April 2016 an approach from an eastern direction was chosen, using a multicopter as carrier for gas sampling and sensing instruments.The take-off area was most of time at the northern shelter (see Fig. 1).Atmos.Meas.Tech.Discuss., https://doi.org/10.5194/amt-2017-335Manuscript under review for journal Atmos.Meas.Tech.Discussion started: 28 November 2017 c Author(s) 2017.CC BY 4.0 License.

2 Masaya
Masaya volcano (elevation ~ 600 m a.s.l.), Nicaragua, is a basaltic-andesite shield volcano caldera (6 x 11 km in size) hosting a set of vents.The currently active vent is situated in the Santiago pit crater, formed in 1858-1859(McBirney, 1956)).Masaya persistently emits voluminous quantities of SO2, with fluxes typically ranging from 500 T/d to 2500 T/d (e.g. de Moor et al., 2013;Carn et al., 2017), making this volcano the single largest contributor to volcanic gas emissions in the Central American Volcanic Arc (Mather et al., 2006).In January 2016 the reoccurrence of a new superficial lava lake (~40 x 40 m) was observed together with an increase in activity.Due to high emission rates and the low-altitude plume, Masaya volcano has a detrimental environmental impact on the downwind areas, diminishing vegetation and potentially affecting human health (Delmelle et al., 2002).Continuous monitoring of the gas emissions is realized by a stationary Multi-GAS (MG) system (through the DCO-DECADE program) at the crater rim and two scanning DOAS instruments (NOVAC network (Galle et al., 2010)) in the downwind direction.Besides the presence of a strong plume, Masaya volcano provides perfect conditions for field-testing new methods and studying plume chemistry using UAVs: easy accessibility by car, low altitude, and relative stable dominant wind direction (northeast).In July 2016 flights were launched from the caldera bottom marked "flight area" in Fig. 2 (c).

3 Turrialba
Turrialba is a stratovolcano with a peak elevation of 3340 m a.s.l. and is located about 35 km east and directly upwind of San José, the capital of Costa Rica.It is the southernmost active volcano of the Central American Volcanic Arc.In the 2000s, an almost 150 years long period of quiescence has ended and since 2010 several vent opening phreatic eruptions henceforth occurred marking an ongoing but erratic phase of unrest (Martini et al., 2010), characterized by variable ash and gas emission intensities (up to 5000 tons/day of SO2 (de Moor et al., 2016a)).The proximity of Turrialba volcano to the densely populated central valley with Costa Rica's major international airport and the dominant western wind direction is responsible for ash depositions, causing health and air traffic problems.Therefore, the activity of Turrialba is continuously monitored by various systems including permanent Multi-GAS stations to observe short-term precursory changes in the gas composition prior to eruptive events (de Moor et al., 2016a).However, stations located near the active vent suffer from ash deposition during more frequent episodes, making maintenance risky or impossible.Furthermore, the accessibility of the summit and surrounding areas is degrading due to intense erosion following vegetation destruction by acid rain, heavy rainfall, ash deposition and remobilization, and the lack of infrastructural maintenance following community evacuation.Thus, the use of UAV-based systems might represent the only viable approach for in-situ measurements of the open vent plume during periods of high activity.In 2016 flights were conducted starting at the "La Silva" site (Fig. 2 (d)) to investigate the feasibility of UAV-based gas sensing system at this challenging environment (high altitude and thick ash plumes).

Unmanned aerial vehicle
The UAV (called RAVEN, remote-controlled aircraft for volcanic emission analysis, see Fig. 3 (a) -(b)) used during the field campaigns was a four-rotor multicopter with foldable arms (Black Snapper, Globe Flight, Germany) with an E800 motor set using propeller with a diameter of 13 inch and a pitch of 4.5 inch (DJI Innovations, Shenzhen, China).It was flown manually in line-of-sight conditions.The multicopter had a weight of 2.3 kg including a 22.2 V (6S) 4.5 Ah battery.A maximum payload of 1.3 kg was achieved with various mounted instruments.The foldable frame of the UAV was beneficial in regards of the usual necessity to personally carry equipment into field, especially on volcanoes like Stromboli.Another advantage of this system was that the battery capacity is within the guidelines of air travel restrictions allowing the system to be transported on commercial airplanes.The main controller (NAZA M-2, DJI Innovations, Shenzhen, China) of the multicopter was connected with a combined denuder sampling and SO2 sensing instrument (hereafter named Black Box (see Sec. 3.3)), to transmit the measured SO2 data as an 0 V -5 V signal to the remote control, where it is displayed.This allowed the operator to find areas with dense plume to hover the system for stationary sampling and to react to changes of the plume direction, which is challenging if relying only on visual observation of the plume.A data logger (Core 2, Flytrex Aviation, Tel Aviv, Israel) with micro SD card was used to log the flight data from the main controller consisting of GPS coordinates, pressure and temperature data at 2 Hz.The payloads were attached below the main body of the multicopter with an inlet for the in-situ CO2 and SO2 sensing instrument (hereafter called Sunkist (see Sec. assumed to originate from within a radius of a few meters (see e.g.Roldan et al., 2015;Alvarado et al., 2017;Palomaki et al., 2017), which represents homogeneous conditions for a widely spread out plume.This assumption is confirmed by a selfdeveloped method for the estimation of the origin of sample air, which is described in the supplementary material (Chapter IV).

CO2/SO2 gas sensors -Multi-GAS (Sunkist)
A gas sensor system was developed for use in volcanic environments with a focus on robustness and compact and lightweight design for application on UAVs.The system, called Sunkist (SK) contains two gas sensors: (1) A CO2 sensor (K30 FR, SenseAir, Delsbo, Sweden), which uses the principle of non-dispersive infra-red absorption spectroscopy: The sample gas gets sucked into a multi reflection cell, where it is exposed to the radiation of a small light bulb.The CO2 concentration is determined by measuring the light attenuation on distinct CO2 absorption bands in the near infra-red.(2) An electrochemical SO2 sensor (CiTiceL 3MST/F, City Technology, Portsmouth, United Kingdom), which basically consists of an electrochemical cell, where oxidation of SO2 on one of the cell's electrodes creates charges and leads to a measureable compensating current between the two cell electrodes.The CO2 sensor is placed inside a hermetic box, which is part of the air path (Fig. 4 (a)).It is equipped with further on-chip sensors for humidity (SHT21 from Sensirion, Staefa, Switzerland), temperature and pressure (BMP180 by Bosch Sensortec, Reutlingen, Germany), which allow to correct the gas data for dependencies on named environmental parameters.
The sensors are read out by a custom-built Arduino Uno Rev 3 computer with a micro SD card logger at a sampling rate of 2 Hz and powered by a rechargeable 3.7 V lithium polymer (LiPo) battery (Fig. 4 (b)).The SO2 sensor was powered by a separate 9 V alkaline battery.The whole system is sheltered in a polystyrene foam case with a total weight of 500 g (Fig. 3 (c)).
A 45-µm pore size PTFE filter was attached to the inlet to prevent particles from entering the sensors.Gas was pumped through to the in-series connected sensors (1.SO2, 2. CO2) by a small pump using a flow rate of 500 ml/min.The system was calibrated before and after field deployments with CO2 (0-1500 ppm) and SO2 (0-30 ppm) test gases.Detailed information on the specifications of the sensors is shown in Table 1 and in the supplementary material (Fig. S1).

Gas diffusion denuder sampler (Black Box)
An in-situ gas sampling system was constructed to enable gas diffusion denuder sampling in the plume at various distances from the vent using the UAV.To compensate for dilution, a CiTiceL 3MST/F SO2 sensor (City Technology, Portsmouth, United Kingdom) was implemented to obtain halogen/sulfur ratios combining denuder samples and sensor data.The sampler (called Black Box (BB)) consisted of the following components, which are introduced in the order the gas passes through: I) inlet system for three denuders; II) electrochemical SO2 sensor; III) mass flow sensor; IV) micro gas pump (see Figure 4 (b) and Table 1).The housing was made of polystyrene foam to ensure weight requirements.An Arduino microcontroller (Uno Rev 3) for signal processing and data logging on a SD card was built in.Various 11.1 V LiPo batteries (500 and 1000 mAh) supplied the power, with different capacities depending on the desired payload and operation time.The Arduino computer transmitted a pulse width modulated signal between 0 V and 5 V, proportional to the detected SO2 mixing ration, to the main controller of the multicopter via cable connection, which then was sent by telemetry to the remote control to allow the operator to assess plume strength in real time to optimize denuder exposure time.The SO2 gas sensor was calibrated in the laboratory and close to field conditions with SO2 gas standards in N2 (0 -54.1 ppm).

Drone-operated miniature differential optical absorption spectroscopy (DROAS)
A miniature UV spectrometer system (Galle et al., 2003) was employed for flying mobile DOAS traverse measurements to conduct estimations of the SO2 flux at Turrialba volcano.This system consisted (Fig. 8 (a)) of an UV spectrometer (USB2000+, Ocean Optics, USA), a miniature telescope (Ocean Optics 74-DA collimating lens, diameter: 5 mm, focal length 10 mm), a GPS Antenna (BU-353-S4, GlobalSat, Taipei, Taiwan) and a miniature on-board computer (VivoStick TS10, ASUS, Taipei, Taiwan).The system was powered by a 11.1 V LiPo battery (1000 mAh), which was connected via a switching regulator (CC BEC 10 A, Castle Creations, USA) to give 9 V at 2 A. The spectrometer and the GPS antenna were connected (and powered) at the computer via USB ports.The NOVAC software "mobile DOAS" developed at the Chalmers University (Sweden) was run on the computer for data acquisition and later evaluation.The miniature PC in the spectrometer system was accessed via a remote desktop connection by a different computer to initialize the data acquisition by the "mobile DOAS" software.
Evaluation of the SO2 fluxes obtained by DROAS traverses was achieved by a comparison with the SO2 fluxes derived by two NOVAC stationary DOAS instruments (La Silva and La Central, see Fig. 2 (d)) located in proximity to the flight area.

Sensor calibration
All three in-situ gas sensors (two identical SO2, one CO2) were calibrated using test gas standards mixed with nitrogen, using either tedlar bags, dynamic dilution or readily mixed test gases.The sensors were exposed to different mixing ratios by pumping the gas mixes through the system.Calibration functions were fitted, including errors in mixing ratio and signal (York et al., 2004) to give sensitivities (slope) and offset levels (intercept) (Supplementary Material Fig. S2 -S4).As the CO2 sensor responds to the gas concentration (molecules per volume), CO2 mixing ratios (molecules per molecules of air) were obtained by compensating the concentration signals with pressure and temperature data recorded by the built in sensors, assuming ideal gas behavior.The SO2 sensor output however, relates directly to the mixing ratio, due to the diffusivity of the transport signal got maximized for discrete peaks (see Fig. 5 (a)).

Gas diffusion denuder analysis
The sampled gas diffusion denuders were sealed air tight and stored in darkness for subsequent analysis.The coating, which contained the derivate and derivatization agent in excess (TMB or EP), was eluted off the denuders using 5 times 2 mL of 1:1 of ethyl acetate and ethanol (in case of TMB) or 5 times 2 mL methanol (EP).After the elution, the solvent was evaporated (at 35 °C under gentle N2 gas stream) to a volume of approximately 100 µL.Internal standards (with TMB: 2,4,6-tribromoaniline; with EP: neocuproine) were added to each sample solution to account for evaporation losses.The condensed samples were analyzed by GC-MS (TMB) and LC-MS (EP) and quantified using external calibration.Due to the lack of a pure calibration standard hydrogen bromide was only measured qualitatively.Halogen mixing ratios in air were derived from the measured halogen amounts on the denuders and the respective sampling volume obtained by the sampling pump data.In addition to the actual sample denuders open field blank denuders were prepared to account for potential diffusive gas precipitation during the flights.

Gas ratios
For a feasible data interpretation, gas ratios were calculated from the sensor data and the denuder analysis results.Halogen mixing ratios were interpreted concerning dilution with ambient air by relating to SO2, which is rather slow in its oxidation, and therefore can be treated as a stable dilution proxy for a short term plume observation (Porter et al., 2002).Thus, the derived halogen mixing ratio was divided by the time-integrated SO2 mixing ratios obtained during the denuder sampling period to obtain BrX/SO2 ratios.Due to a baseline drift in the CO2 sensor data, which was only observable during long-term measurements (> 45 minutes), the pressure and time-response corrected CO2 data was additionally drift corrected for longterm measurements.To do so, a linear fit was made to the sloped background signal and subtracted from the CO2 signal (see supplementary material Fig. S5).CO2/SO2 ratios were calculated with a linear fit to CO2 vs. SO2 scatter plots, considering the deviations (York et al., 2004) of the two sensor signals.

DROAS evaluation and gas fluxes
SO2 emission rates during the flights at Turrialba were derived by traversing the plume with the DROAS instrument pointing vertically upwards in the direction of the plume.The "mobile DOAS" software developed at Chalmers University was used to control the spectrometer and to retrieve SO2 column amounts from the spectra.The SO2 columns were achieved using a wavelength evaluation window of 310-330 nm and including O3 and SO2 absorption cross sections convoluted with a slit function as well as a Ring Spectrum and a 3 rd order polynomial in the DOAS fitting routine.The calculation of the SO2 emission rate involves the integration of the SO2 column amounts measured along the flight path resulting in a cross-sectional SO2 area, which was geometrically corrected to obtain a surface that is orthogonal to the plume direction, and then multiplied by the wind speed (obtained from the NOAA National Center for Environmental Predictions (NCEP) Global Forecast System) to 5 calculate SO2 fluxes.Further details on the spectral evaluation routines and flux calculations can be found elsewhere ( de Moor et al., 2017).

Multicopter performance assessment
During the deployment at three volcanoes, the RAVEN multicopter conducted more than 50 flights under moderate wind conditions, in most cases below 10 m/s.The multicopter achieved a maximum operation altitude of 3320 m at Turrialba volcano and records showed a maximum speed of 85 km/h.With a takeoff weight of 2.45 kg and a payload of maximum 1.3 kg flights at Turrialba volcano were still possible, although the flight time was reduced to about 5 to 8 minutes.At the Masaya volcano sites (takeoff altitude ~ 500 m), a maximum ascent above ground level 1080 m was recorded.A typical flight time at this site was between 10 and 15 minutes.The telemetrically transmitted SO2 mixing ratios from the Black Box apparatus allowed the localization of high plume densities and therefore adjustments on the optimal hover location.While flying in the line of sight the system could be reliably controlled within a distance of 1-2 km to the operator.However, entering the dense plume proved to challenge the connection between remote control and receiver, resulting in multiple connection losses during flights, in which the UAV also left the line-of-sight of the operator.Nevertheless, GPS connection was still present and an automated return mechanism allowed regaining control after the UAV left areas of high plume densities.At Masaya this phenomenon was observed mostly in a plume with condensed water and SO2 mixing ratios in low one-digit ppmv numbers, while in a plume without condensation close to the crater control was maintained even with SO2 levels up to 40 ppmv.This is probably due to the attenuation effect of fog and cloud droplets on millimeter-waves, as they are used with the 2.4 Ghz transmitter of the remote control (Zhao and Wu, 2000).

CO2/SO2 gas sensors
During two field deployments at Stromboli (Table 2) and Masaya (Table 3) volcanoes, the lightweight gas monitoring system Sunkist (SK) determined CO2/SO2 gas ratios at various airborne and ground-based locations.A comparison with a stationary Multi-GAS (MG) instrument at Masaya volcano for the same site and time (lookout point south, 14 th July2016, 11:19) and with inlets of both systems in proximity to each other gave results on CO2/SO2 ratios that were within each other's 2-sigma confidence intervals (CO2/SO2 of 2.9 ± 0.3 for MG and 3.6 ± 0.4 for SK).The time series of the SO2 mixing ratios of SK and MG also showed a good agreement (R² = 0.89) (Fig. 6).An application of SK further downwind (0.5 km from the rim) gave a CO2/SO2 ratio of 3.3 ± 1.2, while the MG measured a CO2/SO2 ratio of 3.1 ± 0.1 during the same time at the rim.This is showing SK's ability for deployment in a more diluted plume, although with the disadvantage of higher errors, due to the higher relative background in CO2 at more distant locations.Furthermore, UAV-based application (9 flights, 17 th -20 th July 2017) between 1.5 km and 2 km downwind of the Masaya plume resulted in average 42 % higher CO2/SO2 ratios with a larger standard deviation compared to the crater rim (14 th -16 th July 2017), but still within each other's errors (CO2/SO2: 5.4 ± 2.3 at 1.5 -2 km; 3.8 ± 0.3 at the rim).Due to the limitations of the CO2 sensing, acquirement of useful CO2/SO2 data with SK is more feasible in dense plumes close to, but not limited to the crater rim, as the airborne application has shown.Additionally, SO2 mixing ratios were measured as a plume dilution proxy by the Black Box (BB) system.Both the BB and SK systems use an identical SO2 sensor and showed a good agreement of their SO2 time series and time integrated SO2 mixing ratio (SK: 1.75 ± 0.08 ppmv, BB: 1,84 ± 0.08 ppmv) (supplement material S6).
At Stromboli volcano, the Sunkist and Black Box systems were deployed on two days (05 th & 06 th April 2016), resulting in seven flights into the plume.These flights covered distances, between 11 m and 419 m from the vent in a downwind direction (northeast), above the Sciara del Fuoco.As shown in Figure 1, discrete gas masses with different CO2/SO2 compositions were measured.The retrieved CO2/SO2 ratios ranged between 7 and 64, with the higher values typically detected directly above the vent (11 m to 26 m) (see Tab. 3 and Fig. 1).During the multicopter operations close to the vent regular strombolian ash explosion occurred (see Fig. 3 (d) and (e)), which are likely accompanied by CO2-rich gas masses (La Spina et al., 2013) and therefore a possible explanation for the detected high CO2/SO2 in a rather undiluted plume region.
On both flight days the predominant wind direction was southwest (207° ± 15° and 210° ± 18°, data from weather station at 77 m a.s.l., commercially available from windfinder.com,Kiel, Germany), which resulted in the plume mostly being present across the Sciara del Fuoco and therefore only accessible by UAV.Nevertheless, a local Multi-GAS station (placed on the SE rim of the crater terrace) has discontinuously measured the plume during the period of the UAV survey, showing CO2/SO2 ratios between 2.2 and 13.6, which is in agreement with some of the multicopter-based measurements.Similar CO2/SO2 ratios have been observed in the past and are exemplary for an ordinary Strombolian activity (Aiuppa et al., 2009).It has to be taken into account that both instruments have not measured simultaneous or in proximity to each other.Furthermore, a comparison of SK and MG CO2/SO2 data might be more accurate with the high SK CO2/SO2 values (associated with eruptive degassing) left aside, as we lack the observations on passive, respectively eruptive degassing behavior for the MG data records.

Halogen measurements
Bromine species were detected by gas diffusion denuder sampling on three of the seven flights at Stromboli volcano.Reactive bromine species were measured between 0.14 and 0.65 ppb (see Table 4), while HBr was determined qualitatively in all three samples.
The obtained ratios of reactive bromine (BrX) to SO2 (1.9*10 -4 -9.8*10 -4 ) are within the range of bromine to sulfur ratios particulate bromine (HBr(aq)), interpretation of the BrX/SO2 ratio is difficult as the emitted gas composition may also change on shorter time scales (La Spina et al., 2013) compared to the campaign duration.However, an increase of the reactive bromine species BrO with distance from the crater rim has been observed previously by DOAS measurements at various volcanoes and is well described in the literature (Oppenheimer et al., 2006;Bobrowski et al., 2007).Although the bromine species in volcanic plumes has been subject of several ground-based (e.g.Gliß et al., 2015) airplane (e.g.General et al., 2015), satellite (e.g.Theys et al., 2009;Hörmann et al., 2013) and model studies (e.g.Bobrowski et al., 2007;Roberts et al., 2009;von Glasow, 2010;Roberts et al., 2014;Jourdain et al., 2016) in recent years, in-situ measurements are scarce.The here presented data for the first minute after emission highlights the potential of UAV-based measurements to improve sample acquisition and thus a better understanding of plume aging.
As shown in Figure 7 (c), the BrX to SO2 ratio seems to change not only with the plume age but also with CO2/SO2 mixing ratios, which were simultaneously measured.However, with only three data points a further interpretation is inadequate due to lack of a statistical basis.Nevertheless, these first results show the principal practicality of the used denuder sampling and gas sensing methods for simultaneous investigation of halogen, carbon and sulfur emissions.

Figure 3 :
Figure 3: (a) Sampling site at Stromboli Volcano with Northeast crater in the background and the RAVEN UAV carrying the Sunkist gas monitor, (b) RAVEN UAV with the Black Box sampling unit and three gas diffusion denuders, (c) interior view on the Sunkist gas monitor, showing the CO2 and SO2 sensors, (d) passive degassing (white plume) and eruptive ash explosion (brown ash cloud) at the north east crater at Stromboli Volcano, (e) ash eruption at the Northeast crater producing an ascending ash plume with the RAVEN UAV in direct proximity (yellow circle), returning from sampling flight Atmos.Meas.Tech.Discuss., https://doi.org/10.5194/amt-2017-335Manuscript under review for journal Atmos.Meas.Tech.Discussion started: 28 November 2017 c Author(s) 2017.CC BY 4.0 License.
Atmos.Meas.Tech.Discuss., https://doi.org/10.5194/amt-2017-335Manuscript under review for journal Atmos.Meas.Tech.Discussion started: 28 November 2017 c Author(s) 2017.CC BY 4.0 License.membranebeing inversely proportional to the pressure and therefore cancelling out the pressure dependency of the concentration.The two gas sensors operate with significantly different response times (T90 for CO2 ~ 2 s, for SO2 ~ <20 s), since sample gas enters the SO2 sensor only by molecular diffusion, whereas in the CO2 sensor it is directly driven through the optical cell.To adapt the response times of the two sensors, the CO2 signal was smoothed through convolution with a first order transfer function.The transfer function's response time factor was chosen, such that the correlation of CO2 and SO2 Atmos.Meas.Tech.Discuss., https://doi.org/10.5194/amt-2017-335Manuscript under review for journal Atmos.Meas.Tech.Discussion started: 28 November 2017 c Author(s) 2017.CC BY 4.0 License.

Figure 7 :
Figure 7: (a) Measured CO2/SO2 gas ratios over a 5-day period by a Multi-GAS station (located east of the crater terrace) and airborne measurements with the Sunkist system, data point shapes indicate the lower SO2 mixing ratio limits for the linear fit over the CO2 vs SO2 data, (b) development of the gaseous reactive bromine species over SO2 over the estimated plume age (derived from distance to crater and estimated wind speed), (c) gaseous reactive bromine/SO2 ratios vs. CO2/SO2 ratio 4.4.DROAS measurements On September 27 th , two DROAS plume transects were performed at Turrialba volcano during mild ash emission at around 2800 m a.s.l. with a total flight time of approximately 10 minutes.The flight was conducted in the downwind direction west of the active vent and crossed underneath the plume on a south-north axis (Fig. 8 (b)), exiting the plume on both ends of the flight path, which is indicated by the low SO2 column amounts close to the baseline in the northern-and southern-most section of the flightpath (Fig. 8 (c)).SO2 fluxes were calculated for that flight and resulted in 1776 ± 1108 T/d SO2 for traverse A and 1616 ± 1007 T/d SO2 for traverse B (see Table5).Calculation of the SO2 fluxes obtained by the two scanning DOAS instrument from the NOVAC network, located on the southern edge of the plume, gave average results of 1533 ± 986 T/d SO2 for La Silva and 1094 ± 704 T/d SO2 from La Cetral for a 2-hour period, in which the flight took place.Therefore, the results of the DROAS traverses are in a good agreement with the scanning DOAS stations.De Moor et al.(de Moor et al., 2016a) conducted car based mobile DOAS transects, which typically take ~45 minutes for a round trip due to rough terrain.Dynamic